In Seattle landscape architect Richard Haag pushed for years to salvage decaying ruins, finally opening Gas Works Park in 1975.

Berlin’s Tempelhof airport became a refuge in 2010. An even more ambitious initiative had started three decades earlier, when Germany focused on rejuvenating its rust belt, in the Ruhr region, by preserving and celebrating that heritage with 50-mile-long Emscher Landscape Park. More than 100 projects are united in the park—factories and other sites have been reclaimed for leisure, cultural events, and green space.

“The Germans were leading practitioners of creative industrial land re-use,” says Timothy Beatley, a “biophilia” expert who sees green space as essential to urban planning and design. At Emscher, nature has been allowed to return, engulfing mills and other structures. “Inside it’s remarkable, almost like a Mayan ruin,” he says.

As urban areas continue to grow, Beatley says, “we need more compact and dense cities but we also want to find places of respite and leisure and connection with nature.”

“Almost any city has a old rail,” says Beatley. “It’s a wonderful opportunity to repurpose space and integrate nature.” Cities are not only tackling train tracks to create parks but also parking lots, highways, bridges, and, in Tel Aviv, even a landfill.

See our picks of innovative transformations in the gallery.

This article is part of our Urban Expeditions series, an initiative made possible by a grant from United Technologies to the National Geographic Society.